Simulating the cleft ion fountain at polar perigee altitudes

A dynamic fluid semi-kinetic (DyFK) model is used to simulate the cleft ion fountain. Ion field-aligned flows are modeled for a flux tube convecting along an empirical model specified convection trajectory across the polar ionosphere from the cusp/cleft region. In this DyFK model, the collision-dominated portion of the flux tube is treated with a moment-based fluid model for altitudes from 120 to 1100 km, while a generalized semi-kinetic model is used for the 800 km to 3 RE altitude region. Convection-driven frictional ion heating and the effects of cusp/cleft soft electron precipitation in the F region/topside ionosphere, and centrifugal acceleration of ions and wave-driven transverse ion heating at high altitudes, are incorporated into the present simulations of the ion field-aligned transport. The modeled evolution of the O+ flow parameters at 5500 km altitude exhibits typical patterns for the cleft ion fountain: O+ field-aligned flows are upward over the dayside cusp/cleft and remain upward for 5 or more degrees latitude into the polar cap, with decreasing magnitude towards the pole; the flows turn downward at about 86° invariant latitude and tend to increase in magnitude (downward) across the polar cap from dayside to nightside; the O+ density also displays apparent day-night asymmetry with higher density on the dayside. The simulated field-aligned flow pattern is in qualitative agreement with the observations from the thermal ion dynamics experiment (TIDE) during a Polar satellite southern perigee pass. The simulated ion densities and field-aligned fluxes are in general consistent with those observed. It is also shown in systematic simulations that the day-night asymmetry of the O+ density across the polar cap from dayside to nightside may be directly controlled by the cleft ion fountain, while the H+ density asymmetry is probably caused by day-night variations in solar illumination.